The invention relates to a hydraulic continuously variable speed transmission composed of a hydraulic pump and a hydraulic motor, and more particularly to a control apparatus for such variable speed transmission mounted on a vehicle.
There have been proposed various types of hydraulic continuously variable speed transmissions (see, for example, Japanese Patent Laid-Open Publication No. 56(1981)-95722); wherein the transmission comprises a constant displacement hydraulic pump driven by an engine and a variable displacement hydraulic motor driven by hydraulic pressure supplied through a hydraulic circuit from the hydraulic pump to transmit the rotation to the wheels. The transmissions are adapted to continuously vary the transmitted engine speed to the wheels by controlling the displacement of the hydraulic motors.
In such hydraulic continuously variable speed transmissions, the driving torque acting on the wheels depends on the hydraulic pressure generated in the hydraulic circuit and on the displacement of the hydraulic motor (i.e., the reduction ratio). This hydraulic pressure in the hydraulic circuit is limited only by a relief valve within a range below a given maximum pressure (relief hydraulic pressure). If the pressure is below this relief hydraulic pressure, the hydraulic pressure in the hydraulic circuit is determined in accordance with the driving load of the wheels.
Therefore, when for example, driving on a road having a low frictional coefficient, if the torque on a wheel exceeds the driving load (the gripping force of wheels) that is determined by the frictional coefficient between the road and the wheel, the wheel receiving that driving torque will slip and the driving (gripping) force thereof will be reduced, and as, a result, the vehicle will not be able to acquire necessary driving force or acceleration. This problem occurs not only in driving the wheels by the engine but also in decelerating the vehicle with engine braking.